That shutdown is likely to drive up the number of cancellations significantly, according to Flight Aware, which tracks airline flights. More than 11,000 flights were cancelled in Houston due to Hurricane Harvey, according to a Flight Aware tally. United was particularly hard-hit, because of its hub in Houston, and the city bore the brunt of the storm.

Airlines' decision to cancel flights days before a storm strikes and even for the days after it moves out of the area might seem like an added stress for travelers, but it's by design.

Public image is part of the reason.

"They feel like they'll get fewer complaints if they pre-emptively cancel flights," said George Hobica, founder of Airfarewatchdog.com, which tracks airfares. "It's better to have people at home, rather than at the airport. It's certainly bad for publicity when you're showing lines of stranded passengers, when you have passengers on cots."

Airlines also need to ensure that their expensive planes — new Boeing 737s cost more than $100 million — are in other cities, out of harm's way.

They also need to position their crew out of the stalled airport and in cities where flights can resume once airports are reopened.

A spokesman for Miami International Airport said the airport will not close but noted that it is not a shelter so an influx of stranded people could stretch basic services. Even if the main terminals are open, the air traffic control tower will order planes to stop operating if winds reach sustained speeds of 55 miles per hour.

U.S. airlines remaining after a wave of megamergers over the past decade have left travelers with fewer options, even though these airlines have, as a result, larger fleets and access to more hubs to reposition aircraft in the event of a storm.

The exact cost of cancelling a flight varies depending on the size of the aircraft, number of crew members and number of paying passengers. United said on Wednesday that Hurricane Harvey likely contributed to a $400 million hit to sales this quarter.